1. Field of the Invention
The present invention relates to a lithium-titanium complex oxide suitable as an electrode material for lithium ion secondary battery, as well as a manufacturing method thereof.
2. Description of the Related Art
Development of lithium ion secondary batteries as high-capacity energy devices has been active in recent years, and lithium ion secondary batteries are beginning to be utilized in consumer equipment, industrial machinery, automobiles and various other fields. Characteristics required of lithium ion secondary batteries include high energy density, high power density and other characteristics that support high capacity and allow for quick charge/discharge. On the other hand, incidents of fire involving a lithium ion secondary battery have been reported and the market is demanding greater safety of lithium ion secondary batteries. In particular, lithium ion secondary batteries used in onboard applications, medical applications, etc., directly affect human life in case of accidents and require even greater safety. Safety is also required of materials used for lithium ion secondary batteries, where, specifically, the market is demanding materials that demonstrate stable charge/discharge behaviors and will not burst or ignite even in unforeseen accidents.
Lithium titanates are expressed by, for example, Li4Ti5O12, Li4/3Ti5/3O4 or Li[Li1/6Ti5/6]2O4, and have a spinel crystalline structure. The aforementioned lithium titanate changes to a rock-salt crystalline structure as lithium ions are inserted during charge, and changes back to a spinel crystalline structure again as lithium ions dissociate. The lithium titanate undergoes far less change in its lattice volume due to charge/discharge compared to carbon materials that are conventional materials for negative electrodes, and generates little heat even when shorted to the positive electrode, thereby preventing fire accidents and ensuring high safety. Lithium-titanium complex oxides whose main constituent is lithium titanate and to which trace constituents have been added as necessary, are beginning to be adopted by lithium ion secondary battery products that are designed with specific focus on safety.
Patent Literature 1 discloses the ambience control method in the sintering step. According to the method described, synthesis reaction is implemented in the sintering step and preliminary sintering step before the sintering step under a flow of nitrogen gas of lower partial oxygen pressure. The purpose of this is to suppress lithium volatilization loss during heat treatment. Patent Literature 2 discloses sintering in an inert gas flow based on low partial oxygen gas pressure. The purpose of this is to obtain a powder of high crystallinity. Both literatures do not provide details of ambience control and only mention that the ambience in question is a sintering ambience.